Question

By reading the std::vector reference I understood that

1. calling insert when the the maximum capacity is reached will cause the reallocation of the std::vector (causing iterator invalidation) because new memory is allocated for it with a bigger capacity. The goal is to keep the guarantee about contiguous data.

2. As long as I stick below the maximum capacity insert will not cause that (and iterators will be intact).

My question is the following:

When reserve is called automatically by insert, is there any way to control how much new memory must be reserved?

Suppose that I have a vector with an initial capacity of 100 and, when the maximum capacity is hit, I want to allocate an extra 20 bytes.

Is it possible to do that?

Answer 1

You can always track it yourself and call reserve before it would allocate, e.g.

static const int N = 20 // Amount to grow by
if (vec.capacity() == vec.size()) {
  vec.reserve(vec.size() + N);
}
vec.insert(...);

You can wrap this in a function of your own and call that function instead of calling insert() directly.